This invention relates to a liquefaction process and apparatus.
In the liquefaction of natural gas with a refrigerant it is known to try to match the natural gas cooling curve with the refrigerant warning curve by splitting the refrigerant into two streams which are cooled to different temperatures. This is described, for example, in ore WO-A-9527179.
In our WO-A-9713108 there is disclosed a compact LNG plant for use in the offshore liquefaction of natural gas. FIG. 1 of the attached drawings illustrates a natural gas liquefaction apparatus of the general type disclosed in WO-A-9713108, although there are differences between FIG. 1 and the disclosure of WO-A-9713108.
In FIG. 1 pretreated natural gas is fed via a conduit 101 to a heat exchanger 166 at a pressure of about 8.3 MPa. In one example, the natural gas in conduit 101 would have the following composition: 4.2 mol% nitrogen; 85.1 mol% methane; 8.2 mol% ethane; and 2.5 mol% propane. The natural gas in the conduit 101 is cooled to a temperature in the range about 5xc2x0 C. to 10xc2x0 C. by heat exchange with chilled water, and is discharged into a conduit 102.
The natural gas exiting the heat exchanger 166 is fed to the warm end of a CWHE (coil wound heat exchanger) 150 via the conduit 102. The CWHB 150 comprises a single shell, which houses two separate heat exchanger bundles 151 and 152. The natural gas is cooled in the CWHE 150 by countercurrent heat exchange with a nitrogen refrigerant. The cooled natural gas leaves the CWHE 150 at a temperature around xe2x88x9290xc2x0 C., and is fed to a farther heat exchanger 153 via a conduit 104. the heat exchanger 153 may be an aluminum PFHE (platefin heat exchanger). The natural gas is cooled to a temperature of about xe2x88x92150xc2x0 C. in the heat exchanger 153, and exits the cool end of the exchanger 153 into a conduit 106.
The natural gas in conduit 106 is fed to the warm end of a heat exchanger 154, in which it is cooled to a temperature of about xe2x88x92160xc2x0 C., and it exits the cool end of the exchanger 154 into a conduit 107. The natural gas in conduit 107 is fed to the top of a nitrogen stripper column 157. The column 157 is needed when the nitrogen content of the feed gas is high and the required composition of the LNG product cannot be achieved using one or two stages of flash separation drums. The stripping process is assisted by using the exchanger 154 to provide reboil heat transferred from the natural gas in conduit 106. LNG is fed from the column 157 to a conduit 167, tom where the LNG is fed to the cool end of the exchanger 154. The exchanger 154 warms the LNG to a temperature of about xe2x88x92160xc2x0 C.; the LNG exits the warm end of the exchanger 154 into a conduit 168, through which it is fed back to the column 157.
LNG is fed from the bottom of the column 157 to a conduit 111 and then to a transfer pump 158. The pump 158 pumps the LNG into a conduit 112 and on to a LNG storage tank 186.
The flash gas, which contains methane and a high proportion of nitrogen, exits from the top end of the column 157 to a conduit 109. The flash gas in conduit 109, which is at a temperature of about xe2x88x92167xc2x0 C., is fed to the cool end of a heat exchanger 155, in which the gas is warmed to a temperature of about xe2x88x9240xc2x0 C. The warmed gas is fed from the warm end of the exchanger 155 to a conduit 110, from which it is fed to a multistage fuel gas compressor 180. The compressor 180 has at least four stages of compression with intercooling between each stage using cooling water. The flash gas is compressed in the compressor 180 from just above atmospheric pressure to a pressure which is typically in the range 2.7 to 5.5 MPa, and is then fed to a turbine 173 of a refrigerant compressor 159, as described in more detail below. High fuel gas pressures are required when the turbine is an aeroderivative turbine, owing to the high compression ratios used in such turbines. The fuel gas compressor 180 thus has a significant power requirement, owing to the high discharge pressure and high nitroen content of the gas, such that a gas turbine drive is usually used from economic considerations, rather than an electric motor drive. As described below, the flash gas fed through the conduit 110 is used to provide the bulk of the fuel gas requirements of the liquefaction plant.
The nitrogen refrigeration cycle which cools the natural gas to a temperature at which it can liqueur will now be described. Nitrogen refrigerant is discharged from the warm end of the CWHE 150 into a conduit 132 at a temperature of about 5xc2x0 C. The nitrogen is fed to a multistage compressor unit 159, which comprises at least two compressor stages 169 and 170, with at least one intercooler 171, and an aftercooler 172. The compressor stages 169 and 170 are driven by a gas turbine 173. The operation of the compressor unit 159 consumes almost all of the power required by the nitrogen refrigeration cycle. The gas turbine 173 is driven by the fuel gas derived from conduit 110.
The compressed nitrogen is discharged from the compressor unit 159 into a conduit 133 at a pressure of about 5.1 MPa. The conduit 133 leads to two conduits 134 and 135 between which the nitrogen from the conduit 133 is split according to the power absorbed by the compressor. The nitrogen in the conduit 134 is fed to a compressor 162 in which it is compressed to a pressure of about 8.5 MPa, and is then fed from the compressor 162 to a conduit 136. The nitrogen in the conduit 135 is fed to a compressor 163 in which it is compressed to a pressure of about 8.5. MPa, and is then fed from the compressor 163 to a conduit 137. The nitrogen in both the conduits 136 and 137 is fed to a conduit 138 and then to a heat exchanger 164, where it is cooled to ambient temperatures. The nitrogen is fed from the heat exchanger 164 through a conduit 139 to a heat exchanger 165 in which it is cooled to a temperature of 5xc2x0 C. to 10xc2x0 C. by chilled water. The cooled nitrogen is fed from the exchanger 165 to a conduit 140, which leads to two conduits 120 and 141. The nitrogen flowing through the conduit 140 is split between the conduits 120 and 141: about 2% of the nitrogen in conduit 140 flows through the conduit 141. art The nitrogen flowing through the conduit 141 is fed to the warm end of the heat exchanger 155, where it is cooled to a temperature of about xe2x88x92123xc2x0 C. by countercurrent heat exchange with the flash gas from the column 157. The cooled nitroen is discharged from the cool end of the exchanger 155 to a conduit 142.
The conduit 120 is connected to the warm end of the CWHE 150, whereby the nitrogen is fed to the warm end of the heat exchanger bundle 151. The nitrogen from conduit 120 is pre-cooled to about 13xc2x0 C. in the heat exchanger bundle 151. A majority of the nitrogen refrigerant is withdrawn from the CWHE 150, after passing through the bundle 151, via a conduit 122. The remainder of the nitrogen refrigerant passes through the bundle 152, is cooled to a temperature of about xe2x88x9290xc2x0 C., and is discharged from the CWHE 150 into a conduit 124.
The nitrogen in the conduit 122 is fed to a turbo expander 160, in which it is work expanded to a pressure of about 1.9 MPa and a temperature of about xe2x88x9295xc2x0 C. The expanded nitrogen is discharged from the expander 160 into a conduit 128. The nitrogen in the conduit 124 is mixed with the nitrogen in the conduit 142, and is then fed to a turbo expander 161 in which it is work expanded to a pressure of about 1.9 MPa and a coolest nitrogen temperature of about xe2x88x92151xc2x0 C. The expanded nitrogen is discharged from the expander 161 into a conduit 126. The turbo expander 160 is arranged to drive the compressor 162, and the turbo expander 161 is arranged to drive the compressor 163. In this way the majority of the work produced by the expanders 160 and 161 can be recovered.
The nitrogen in the conduit 126 is fed to the cool end of the heat exchanger 153, and cools the natural gas therein by countercurrent heat exchange. In the heat exchanger 153 the nitrogen is warmed to an intermediate nitrogen temperature of about xe2x88x9295xc2x0 C. The nitrogen exits the warn end of the heat exchanger 153 and is mixed with the nitrogen in the conduit 128 before being fed to the cool end of the CWHE 150. The nitrogen in the CWHE 150 cools the natural gas therein by countercurrent heat exchange.
The heat exchangers 153, 154 and 155, and the column 157 are arranged within a cold box 181.
Inlet combustion air for the gas turbine 173 is fed to a beat exchanger 182 where it is cooled to 5xc2x0 C. to 10xc2x0 C. by heat exchange with chilled water. The combustion air is then discharged into a conduit 183 and is fed to the turbine 173.
Chilling of the inlet air to the gas turbine increases the power output where the ambient air temperature is high.
As in most large scale LNG plants, the most expensive items of equipment are the gas turbine drives and compressors as well as the main CWHE cooling exchangers, such as the bundles 151 and 152 which are normally made of aluminum.
It is an object of the present invention to improve the efficiency and lower the capital cost of prior art processes for liquefying natural gas.
The present invention relates to a method and apparatus for liquefying natural gas, comprising a series of heat exchangers for cooling the natural gas in countercurrent heat exchange relationship with a refrigerant, compression means for isentropically expanding at least two separate streams of the compressed refrigerant, wherein said expanded streams of refrigerant communicate with a cool end of a respective one of the heat exchangers.
One important aspect of the invention involves the use of a precooling refrigeration system to precool the natural gas to a low temperature below 0xc2x0 C., before it is fed to the warm end of the series of heat exchangers. The precooling refrigeration system is also used to precool the high pressure refrigerant to a temperature below 0xc2x0 C. before it is fed to any of the heat exchangers in the series of heat exchangers or to the expansion means. This has been found to reduce significantly power requirements of liquefaction apparatus, and reduce the number of components of the equipment. Advantageously, substantially all the refrigerant in the refrigeration cycle is precooled by the precooling refrigeration system.
It is desirable that the refrigerant in a first of said separate refrigerant streams is cooled in at least one of the series of heat exchangers; this takes place after it has been precooled in the precooling refrigeration system. Furthermore, the use of the precooling refrigeration system makes it unnecessary to cool more than one of the refrigerant streams in the series of beat exchangers, so we prefer that each refrigerant stream other than the first is fed directly to its respective expansion means without farther cooling in the series of heat exchangers.
The refrigerant of the refrigerant streams may be precooled before or after being separated into said streams, although it is more convenient and economical to carry out the precooling before separation. Preferably the refrigerant is split into two refrigerant streams.
We have unexpectedly found that, by using a precooling refrigeration system, it is only necessary to use two heat exchangers in the series of heat exchangers, which is fewer than in WO-A-9527179 and WO-A-9713108, and which leads to significant savings in the cost of manufacturing, operating and maintaining the heat exchangers.
Accordingly, in the preferred embodiment there are two heat exchangers in the series of heat exchangers, the refrigerant is split into first and second refrigerant streams, and only the refrigerant in the first refrigerant stream is cooled in a first, warmest, of said two heat exchangers. Thus, when two refrigerant streams are used, the first refrigerant stream can be fed through the warmest of the heat exchangers in the series of heat exchangers, and the second refrigerant stream can be fed directly to the expansion means without passing through the series of heat exchangers. This allows the heat transfer area in the first heat exchanger to be reduced by about 35% compared with the arrangement shown in FIG. 1, and reduces the complexity of the equipment. This makes it easier to use less expensive types of heat exchanger, such as an aluminum PFHE or a printed circuit heat exchanger (PCHE), instead of a CWHE.
The intercooler 171, shown in FIG. 1, is an expensive piece of equipment because of its high design pressure, large area requirement, and titanium construction used for the parts in contact with sea water cooling medium. With the apparatus of the present invention it is possible to dispense with the intercooler 171 of FIG. 1, because the compressed refrigerant discharged from the compression means is within normal bounds.
Furthermore, the present invention makes it possible to reduce the complexity of the compression means, because the lower refrigerant temperature and, therefore, lower head requirement, allows either a smaller number of compressor wheels or a reduction in wheel diameter; additionally, the number of nozzles required on the compressor case can be reduced from 4 to 2, leading to further cost savings. Another advantage is that the power requirement for the compression means is reduced by about 16% for the same natural gas capacity, which makes it possible to reduce the rating of a turbine used to power the compressor. This makes it possible to replace the two compressor/intercooler arrangement of FIG. 1 with a single compressor stage and no intercooler.
The expanded refrigerant of the first refrigerant stream is preferably fed to a cool end of the second heat exchanger, and the expanded refrigerant of the second refrigerant stream is preferably fed to the cool end of the first heat exchanger. Before being fed to the cool end of the first heat exchanger, the expanded refrigerant of the second refrigerant stream is preferably mixed with the expanded refrigerant of the first refrigerant stream discharged from the warm end of the second heat exchanger.
The two heat exchangers of said series of heat exchangers may be separate, or may be provided in a single heat exchanger shell having two heat exchange bundles therein; each bundle corresponds to one of the heat exchangers of said series of heat exchangers. The use of a single heat exchanger has the advantage that the cold box can be omitted without any significant disadvantageous effects on efficiency.
The natural gas and the refrigerant are preferably precooled in the precooling refrigeration system to a temperature in the range 0xc2x0 C. to xe2x88x9240xc2x0 C., preferably xe2x88x9210xc2x0 C. to xe2x88x9230xc2x0 C. It is preferred to cool the natural gas and the refrigerant to substantially the same temperature with the precooling refrigeration system. The refrigerant is typically discharged from the warm end of the warmest heat exchanger at a temperature below xe2x88x9220xc2x0 C.
An industry standard refrigeration system can be used, to precool the nitrogen and natural gas streams in two or more stages. The number of refrigeration stages for the system is selected depending on the final precooling temperature and by optimising the power requirements of the refrigeration system against the increase in cost for the larger number of equipment items.
There is a range of types of heat exchanger that could be used as the precooling heat exchangers For example, the precooling heat exchanger may be of the aluminum core-in-kettle type or an aluminum plate-fin heat exchanger PFHE or a PCHE. However, it is preferred for economic reasons that the precooling heat exchangers are conventional kettle type shell and rube chillers constructed of carbon steel.
The precooling refrigerant system precools both the natural gas and the natural gas refrigerant using a separate precooling refrigerant. The precooling refrigerant may be, for example, propane, propylene, ammonia or a Freon refrigerant. It is preferred that the precooling refrigerant is R410a Freon, because it is relatively safe and environmentally benign with a high capacity.
In the present invention, the precooling refrigerant is advantageously compressed by a single compression unit comprising two or more compressors stages driven by a precooling gas turbine, instead of using a plurality of separate electric motor driven chiller units as in FIG. 1. A two-stage refrigeration system is usually suitable, but in some cases a three or four stage system may be advantageous The reduction in the overall electric power requirements of the plant brought about by the elimination of the electric motor driven chiller units allows the precooling gas turbine to economically power an electric generator in addition to the compression unit for the precooling refrigeration system. This electric generator can meet all the normal power requirements of the apparatus according to the invention and allows a substantial reduction in the investment required for separate gas turbine driven electric generators required in FIG. 1.
In a preferred embodiment, the natural gas discharged from the series of heat exchangers is fed to a nitrogen stripper column. The natural gas discharged from the series of heat exchangers may be fed to a heat exchanger within the stripper column, at or near the bottom of the stripper column, in order to provide reboil heat for the column; apart from this, it is preferred that the natural gas discharged from the series of heat exchangers is not subjected to any other heat exchange before being fed into the stripper column.
The stripper column generates a gaseous top product containing nitrogen and methane, and it is preferred that this top product is used as a fuel gas to power a turbine for driving the compression means for the refrigerant. The top product is preferably compressed in a fuel gas compressor before being fed to the turbine, and desirably the top product is not subjected to any heat exchange before being fed to the fuel gas compressor.
The arrangement of the stripper column, in accordance with the invention, makes it possible to use a cold box of a smaller size, and containing less equipment, than the cold box 181 in FIG. 1 (or even eliminate the cold box when the series of heat exchangers is provided in a single heat exchanger shell).
Furthermore, by feeding the top product directly to the fuel gas compressor, without any intermediate heat exchange, the suction temperature to the fuel gas compressor is lower, which reduces the power requirements and complexity. With the present invention, the fuel gas compressor may comprises a single compressor or two compressor stages with a single intercooler, and the power requirement can be reduced by up to about 50% compared with the compressor 180 in FIG. 1. Furthermore, an electric motor driven compressor may be used instead of the more expensive gas turbine.
The refrigerant is preferably nitrogen, and it is preferred that the refrigerant in the gaseous phase through the refrigerant cycle.
The relative flow rates of the first and second refrigerant streams can be controlled to match as closely as possible the natural gas cooling curve with the nitrogen warming curve. This is described in more detail in, for example, WO-A-9713108 and WO-A-9527179.
The apparatus according to the invention may be used in an offshore apparatus for liquefying natural gas, as described in WO-A-9713108. In this embodiment, the apparatus can be provided on a support structure (for example a ship) which is floatable or is otherwise adapted to support the apparatus at least partially above sea level.
With the arrangement of FIG. 1 a specific power of about 15.8 KW/tonne LNG produced/day is required. With the apparatus according to the invention a specific power of about 14.75 KW/tonne LNG produced/day is required. It will be appreciated that this is a significant power saving and is additional to the capital cost savings mentioned above.